They are now an instrument of policy. There was a time when a AR lower was "not a handgun" and could be bought out of state. Receivers are now a specifically listed type so they no longer long guns and therefore not allowed for out of state purchase - with no change in law.
Help me understand what you are saying.
18 USC 922(b)(3)
(b)It shall be unlawful for any
licensed importer,
licensed manufacturer,
licensed dealer, or
licensed collector to sell or deliver—
(3)
any
firearm to any person who the licensee knows or has reasonable cause to believe does not reside in (or if the person is a corporation or other business entity, does not maintain a place of business in) the
State in which the licensee’s place of business is located, except that this paragraph (A) shall not apply to the sale or delivery of any
rifle or
shotgun to a resident of a
State other than a
State in which the licensee’s place of business is located if the transferee meets in person with the transferor to accomplish the transfer, and the sale, delivery, and receipt fully comply with the legal conditions of sale in both such
States (and any
licensed manufacturer,
importer or
dealer shall be presumed, for purposes of this subparagraph, in the absence of evidence to the contrary, to have had actual knowledge of the
State laws and
published ordinances of both
States), and (B) shall not apply to the loan or rental of a
firearm to any person for temporary use for lawful sporting purposes;
So the law clearly says the only exception is rifle or shotgun.
18 USC 921(a)(7)
(7) The term “rifle” means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire only a single projectile through a rifled bore for each single pull of the trigger.
18 USC 921(a)(5)
(5) The term “shotgun” means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger.
So the definition of both rifle and shotgun are part of USC and not a CFR. The law seems to cover the case of a receiver (AR lower) not being lawful to transfer across state lines. Had they manipulated a CFR to change definitions I would understand what you are saying.
I can imagine that in the past FFLs did not properly understand the law and would transfer lowers or other non-handguns thinking that is what the law said when it only exempts rifles and shotguns. No doubt this resulted in an ATF clarification/notification to get people back in compliance (or lots of findings during audits)